The invention relates to a method of manufacturing biaxially stretched film from a length of plastic material extruded in tubular form from an extruder. In the initial phase a gaseous pressure medium is introduced into the interior of the tube as the first step, the inflated tube being then compressed locally by means of two sets of squeezing rollers, or nip rollers, running at different peripheral speeds so that an intermediate bubble is formed between the two sets of rollers and two additional bubbles are formed between the extruder nozzle and the first set of rollers and between the second set of rollers and a take-up roller, respectively.
This is a continuation-in-part application of U.S. patent application Ser. No. 284,542 filed Aug. 29, 1972 now abandoned. A method is known in which the gas pressure is an intermediate bubble between two sets of rollers is initially raised to a relatively high value, a gas supply pipe introducing gas at a relatively high pressure into the intermediate bubble after passing into the tube from within the annular extruder nozzle, through the first bubble and then between the rollers of the set of squeezing rollers nearest the extruder nozzle and thence into the intermediate bubble. The pressure thereby obtainable in the intermediate bubble is inadequate, and, in addition, there will be leakages between the gas supply pipe and the rollers of the squeezing roller set nearest the extruder nozzle so that the gas leaks out of the intermediate bubble. This is not satisfactory.
It is the object of the invention to devise a method of the aforesaid kind whereby it becomes possible to readily establish and maintain high pressure in the intermediate bubble so that an adequate stretch ratio is attained in the transverse direction of the plastic film.